Mutations in the Autism susceptibility candidate 2 gene (AUTS2), whose protein is believed to act in neuronal cell nuclei, have been associated with multiple psychiatric illnesses, including autism spectrum disorders, intellectual disability, and schizophrenia. Here we show that cytoplasmic AUTS2 is involved in the regulation of the cytoskeleton and neural development. Immunohistochemistry and fractionation studies show that AUTS2 localizes not only in nuclei, but also in the cytoplasm, including in the growth cones in the developing brain. AUTS2 activates Rac1 to induce lamellipodia but downregulates Cdc42 to suppress filopodia. Our loss-of-function and rescue experiments show that a cytoplasmic AUTS2-Rac1 pathway is involved in cortical neuronal migration and neuritogenesis in the developing brain. These findings suggest that cytoplasmic AUTS2 acts as a regulator of Rho family GTPases to contribute to brain development and give insight into the pathology of human psychiatric disorders with AUTS2 mutations.

In the cerebellum, the bHLH transcription factors Ptf1a and Atoh1 are expressed in distinct neuroepithelial regions, the ventricular zone (VZ) and the rhombic lip (RL), and are required for producing GABAergic and glutamatergic neurons, respectively. However, it is unclear whether Ptf1a or Atoh1 is sufficient for specifying GABAergic or glutamatergic neuronal fates. To test this, we generated two novel knock-in mouse lines,Ptf1a^Atoh1andAtoh1^Ptf1a, that are designed to express Atoh1 and Ptf1a ectopically in the VZ and RL, respectively. InPtf1a^Atoh1embryos, ectopicallyAtoh1-expressing VZ cells produced glutamatergic neurons, including granule cells and deep cerebellar nuclei neurons. Correspondingly, inAtoh1^Ptf1aanimals, ectopicallyPtf1a-expressing RL cells produced GABAergic populations, such as Purkinje cells and GABAergic interneurons. Consistent results were also obtained fromin uteroelectroporation ofPtf1aorAtoh1into embryonic cerebella, suggesting that Ptf1a and Atoh1 are essential and sufficient for GABAergic versus glutamatergic specification in the neuroepithelium. Furthermore, birthdating analyses with BrdU in the knock-in mice or with electroporation studies showed that ectopically produced fate-changed neuronal types were generated at temporal schedules closely simulating those of the wild-type RL and VZ, suggesting that the VZ and RL share common temporal information. Observations of knock-in brains as well as electroporated brains revealed that Ptf1a and Atoh1 mutually negatively regulate their expression, probably contributing to formation of non-overlapping neuroepithelial domains. These findings suggest that Ptf1a and Atoh1 specify spatial identities of cerebellar neuron progenitors in the neuroepithelium, leading to appropriate production of GABAergic and glutamatergic neurons, respectively.

In the cerebellum, all GABAergic neurons are generated from the Ptf1a-expressing ventricular zone (Ptf1a domain). However, the machinery to produce different types of GABAergic neurons remains elusive. Here we show temporal regulation of distinct GABAergic neuron progenitors in the cerebellum. Within the Ptf1a domain at early stages, we find two subpopulations; dorsally and ventrally located progenitors that express Olig2 and Gsx1, respectively. Lineage tracing reveals the former are exclusively Purkinje cell progenitors (PCPs) and the latter Pax2-positive interneuron progenitors (PIPs). As development proceeds, PCPs gradually become PIPs starting from ventral to dorsal. In gain- and loss-of-function mutants for Gsx1 and Olig1/2, we observe abnormal transitioning from PCPs to PIPs at inappropriate developmental stages. Our findings suggest that the temporal identity transition of cerebellar GABAergic neuron progenitors from PCPs to PIPs is negatively regulated by Olig2 and positively by Gsx1, and contributes to understanding temporal control of neuronal progenitor identities.

Climbing fiber (CF) neurons in the inferior olivary nucleus (ION) extend their axons to Purkinje cells, playing a crucial role in regulating cerebellar function. However, little is known about their precise place of birth and developmental molecular machinery. Here, we describe the origin of the CF neuron lineage and the involvement ofPtf1a(pancreatic transcription factor 1a) in CF neuron development. Ptf1a protein was found to be expressed in a discrete dorsolateral region of the embryonic caudal hindbrain neuroepithelium. Because expression of Ptf1a is not overlapping other transcription factors such as Math1 (mouse atonal homolog 1) and Neurogenin1, which are suggested to define domains within caudal hindbrain neuroepithelium (Landsberg et al., 2005), we named the neuroepithelial region the Ptf1a domain. Analysis of mice that express β-galactosidase from thePtf1alocus revealed that CF neurons are derived from the Ptf1a domain. In contrast, retrograde labeling of precerebellar neurons indicated that mossy fiber neurons are not derived from Ptf1a-expressing progenitors. We could observe a detailed migratory path of CF neurons from the Ptf1a domain to the ION during embryogenesis. InPtf1anull mutants, putative immature CF neurons produced from this domain were unable to migrate or differentiate appropriately, resulting in a failure of ION formation. Apoptotic cells were observed in the mutant hindbrain. Furthermore, the fate of some cells in thePtf1alineage were changed to mossy fiber neurons inPtf1anull mutants. These findings clarify the precise origin of CF neurons and suggest thatPtf1acontrols their fate, survival, differentiation, and migration during development.

Cerebellar granule cell precursors (GCPs) and granule cells (GCs) represent good models to study neuronal development. Here, we report that the transcription factor myeloid ectopic viral integration site 1 homolog (Meis1) plays pivotal roles in the regulation of mouse GC development. We found that Meis1 is expressed in GC lineage cells and astrocytes in the cerebellum during development. Targeted disruption of the Meis1 gene specifically in the GC lineage resulted in smaller cerebella with disorganized lobules. Knock-down/knock-out (KO) experiments for Meis1 andin vitroassays showed that Meis1 binds to an upstream sequence of Pax6 to enhance its transcription in GCPs/GCs and also suggested that the Meis1–Pax6 cascade regulates morphology of GCPs/GCs during development. In the conditional KO (cKO) cerebella, many Atoh1-positive GCPs were observed ectopically in the inner external granule layer (EGL) and a similar phenomenon was observed in cultured cerebellar slices treated with a bone morphogenic protein (BMP) inhibitor. Furthermore, expression of Smad proteins and Smad phosphorylation were severely reduced in the cKO cerebella and Meis1-knock-down GCPs cerebella. Reduction of phosphorylated Smad was also observed in cerebellar slices electroporated with a Pax6 knock-down vector. Because it is known that BMP signaling induces Atoh1 degradation in GCPs, these findings suggest that the Meis1–Pax6 pathway increases the expression of Smad proteins to upregulate BMP signaling, leading to degradation of Atoh1 in the inner EGL, which contributes to differentiation from GCPs to GCs. Therefore, this work reveals crucial functions of Meis1 in GC development and gives insights into the general understanding of the molecular machinery underlying neural differentiation from neural progenitors.SIGNIFICANCE STATEMENTWe report that myeloid ectopic viral integration site 1 homolog (Meis1) plays pivotal roles in the regulation of mouse granule cell (GC) development. Here, we show Meis1 is expressed in GC precursors (GCPs) and GCs during development. Our knock-down and conditional knock-out (cKO) experiments andin vitroassays revealed that Meis1 is required for proper cerebellar structure formation and forPax6transcription in GCPs and GCs. The Meis1–Pax6 cascade regulates the morphology of GCs. In the cKO cerebella, Smad proteins and bone morphogenic protein (BMP) signaling are severely reduced and Atoh1-expressing GCPs are ectopically detected in the inner external granule layer. These findings suggest that Meis1 regulates degradation of Atoh1 via BMP signaling, contributing to GC differentiation in the inner EGL, and should provide understanding into GC development.

Neurodevelopmental disorders (NDDs), including autism spectrum disorders (ASD) and intellectual disability (ID), are a large group of neuropsychiatric illnesses that occur during early brain development, resulting in a broad spectrum of syndromes affecting cognition, sociability, and sensory and motor functions. Despite progress in the discovery of various genetic risk factors thanks to the development of novel genomics technologies, the precise pathological mechanisms underlying the onset of NDDs remain elusive owing to the profound genetic and phenotypic heterogeneity of these conditions. Autism susceptibility candidate 2 (AUTS2) has emerged as a crucial gene associated with a wide range of neuropsychological disorders, such as ASD, ID, schizophrenia, and epilepsy. AUTS2 has been shown to be involved in multiple neurodevelopmental processes; in cell nuclei, it acts as a key transcriptional regulator in neurodevelopment, whereas in the cytoplasm, it participates in cerebral corticogenesis, including neuronal migration and neuritogenesis, through the control of cytoskeletal rearrangements. Postnatally, AUTS2 regulates the number of excitatory synapses to maintain the balance between excitation and inhibition in neural circuits. In this review, we summarize the knowledge regarding AUTS2, including its molecular and cellular functions in neurodevelopment, its genetics, and its role in behaviors.